I'd say any given episode of the first two seasons was a chance for Megs to off Starscream. But to be fair, Starscream had his fair share of chances to kill off Megatron which he either didn't take advantage of or was too stupid to pull it off.

I can't help but think that they both secretly get a kick out of their rivalry (for lack of better term) to the extent that their relationship is complimentary. If they were to kill each other off then:
* Megs would have no one to keep him on his toes with all those constant attempts at backstabbing which would in term take some edge off his ruthlessness which would not make him into the character that everyone loves or hates so much.
* SS would have nothing else to aspire to. Once he reaches the top he wouldn't know what to do with himself. Most probably he'd screw everything up for the decepticons and then wonder why he even coveted Megs position in the first case.

Surely Starscream is the epitome of the Decepticon philosophy. Betrayal, conspiracy (if very thinly veiled), and murder in the pursuit of ultimate power. Why would Megatron really want to kill a creation that so embodies his ideals. Also, the Decepticon operate on a system whereby the strongest rules; none of the other Decepticons apart from Starscream have ever been shown to be willing to challenge Megatron and thus allow him to prove his leadership credentials. In such a combative system, having an ever-present and ever-willing challenger to your throne is necessary in order to ensure your continued hegemony.

Haha. yeah in any other tv show in which they weren't giant robots, they would totally be together. Wonder which one would be the girl?

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Did you really need to even ask that question?

Back on subject, in "Hoist Goes Hollywood", Megatron beats the crap out of Starscream and ends up deactivating him after ripping some circuitry right out of his chest. As the observant on the TFWiki have noted, Starscream wasn't seen again for several episodes after that, suggesting that Megatron really messed him up that time.